Hi, I am trying to see if the crazyflie 2 is completely open source. What I am specifically looking for is access at the low-level control commands to be able to develop a control algorithm that modifies the speed of each motor individually. Does the crazyflie 2 supports full open source access to every part of the hardware through ROS?

Also, I want to use Vicon cameras for pose estimation. However, I am not sure if it is possible to place 3 markers on the drone because its payload is only 15 grams and will not take off if the markers are heavier. Has anyone used Vicon cameras and markers previously on the crazyflie 2?

When the nRF51 is compiled without the Bluetooth LE stack, Crazyflie 2.0 firmware is fully open-source. This does not mean that you can have a full low level access to the hardware remotely from ROS: some real-time control can not practically be done remotely and have to be programmed in C in the Crazyflie firmware. My answer to your other thread explains what can currently be controlled remotely viewtopic.php?p=12320#p12304. If you want more, you can implement radio packets and ros support for it. Though as far as I understand ROS itself is not designed as a hard real-time system.

Placing Vicon markers on Crazyflie has been done before, see for example this thesis repos: https://github.com/blandry/crazyflie-tools, the CAD folder contains model of Crazyflie with a marker support structure.